The synthesis, structural characterization, and reactivity of two iron(III) compounds with 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(4-(tert-butyl)benzene-1,2-diol) (tbucatbpy) and 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(4-(tert-butyl)-2-methoxyphenol) (mecrebpy) as ligands under electrochemically reducing conditions is reported. Differences in selectivity and activity for electrocatalytic reduction of carbon dioxide (CO2) to formate (HCO2–) were assessed via cyclic voltammetry (CV) and bulk electrolysis in N,N-dimethylformamide (DMF). These mechanistic studies suggest that the unmetalated O atoms in the secondary coordination sphere are important for relaying exogeneous proton donors to the active site. Depending on the strength of the interaction between the pendent moiety and the sacrificial proton donor, improvements in catalytic activity were observed. Controlled potential electrolysis experiments indicated increased Faradaic efficiencies (FE) for formate (85(2)%) with Fe(mecrebpy)Cl, whereas Fe(tbucatbpy) had a lower FE (70(8)%) in comparison to our previous study containing no pendent relay. On the basis of this data, the equilibrium exchange reactions involving sacrificial proton donor transfer to the pendent O atoms subsequently the Fe active site are better matched in Fe(mecrebpy) than Fe(tbucatbpy), enhancing both the observed catalytic rate constant and selectivity for formate as a CO2 reduction product.

Electrocatalytic CO2 reduction with Fe(III) Schiff base-type complexes containing pendent proton relays

2019 Fall National Meeting

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